Not only does iOS take up about 1 GB of space, the actual disk space varies because of the definition of a GB, and what is advertised as a GB. Apple usually warns that actual disk capacity may very.
The problem is computers and iOS use base 2 (2, 4, 8, 16, 32...) while Apple advertised 32 GB in base 10. It works out that you get around 6.4% less that what is advertised.